This invention relates to a semiconductor laser device, and more particularly to a device used in an optical time domain reflectometer.
An optical time domain reflectometer is used, for example, to test an optical fiber network. A semiconductor laser emits a pulse modulated laser light into an optical fiber under test. The emitted laser light travels in the optical fiber and is reflected from surfaces of discontinuity. If there is a broken point in the fiber, a strong reflection is obtained from the broken point. The reflected light is detected by the semiconductor laser, and by measuring the time difference between the time of emission and the time of detection, the position of the broken point is determined.
Furthermore, there has been a problem of multiple reflections in a heretofore known reflectometer. An echo (reflected light) returns to the reflectometer and enters the radiating surface of the semiconductor laser. A large amount of the energy of the echo entering the surface is absorbed in the semiconductor laser, but a portion (for example about 10%) is reradiated from the surface. If this reradiated energy is above a certain level, echoes of the reradiated light are detected as multiple reflections.
When an optical fiber under test has branch lines, discrimination of multiple reflections from primary reflections becomes difficult, and multiple reflections give false information about broken points. Thus, reflected light returning to the reflectometer must be sufficiently attenuated before going out from the reflectometer as reradiated light. For example, a total attenuation of over 25 dB (including the 10 dB attenuation at the reradiation) is necessary in order to avoid problems from multiple reflections.
A light attenuator on the path of light in the reflectometer attenuates the reflected light, but it also attenuates the emitted light. An attenuation of the emitted light causes a corresponding attenuation of the reflected light and deteriorates the signal to noise ratio at the detection of the reflected light. Therefore, only the reflected light must be selectively attenuated leaving the emitted light intact.
This kind of selective attenuation can be achieved by a light isolator where a polarizer is placed at an input of a Faraday cell which rotates the plane of polarization and a polarization detector at an output of the Faraday cell. However, this type of light isolator has a complex structure and is bulky for use in a compact semiconductor laser device. Moreover, the isolator itself is expensive.